The outer membrane of Gram-negative bacteria consists of a unique molecule known as lipid A that serves as the membrane anchor for lipopolysaccharide (LPS). Lipid A (endotoxin) is the component of LPS responsible for the stimulation of the host innate immune system involved in Gram- negative sepsis. The lipid A of Escherchia coli is a hexa-acylated disaccharide of glucosamine that is substituted at the 1- and 4'- positions with phosphate and glycosylated at the 6' position with two Kdo (3-deoxy-D-manno-octulosonic acid) moieties. Nine enzymes are required for biosynthesis of Kdo2- lipid A, the minimal LPS required for E. coli growth under normal laboratory conditions. Since lipid A is required for bacterial growth, it has become an interesting target for the design of novel antibacterial agents. [unreadable] [unreadable] Although single copies of the lipid A biosynthetic genes are found in nearly all Gram-negative bacterial genomes including those of Helicobacter pylori, the lipid A of the latter is underacylated with the phosphate groups either absent or modified. The primary focus of the present study is the identification of novel enzymes required for the modification of H. plyori lipid A and initial studies to evaluate the importance of such modifications during infection. Secondly, the lipid A structure of Helicobacter heilmannii will be investigated. H. pylori is now considered the causative agent of gastric and duodenal ulcers and H. heilmannii has recently been found associated with human gastritis. The specific aims of the current proposal are: (I) characterization and cloning of lipid A deacylases of H. pylori; (II) characterization and cloning of genes required for modification of the phosphates of H. pylori lipid A; (III) relevance of H. pylori lipid A modifications during infection; and (IV) isolation, purification, and structural characterization of key lipid A species of H. heilmannii. The completion of these aims will not only further the understanding of the lipid A biosynthetic pathway in H. pylori and H. heihnannii but also lay the foundation for new molecular insights into the pathogenesis of these unique organisms. [unreadable] [unreadable]